<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    body {
      padding: 0;
      margin: 0;
      display: flex;
      justify-content: center;
      flex-direction: column;
      width: 100vw;
      height: 100vh;
    }


    div {
      height: 60px;
    }

    iframe {
      flex: 1;
    }
  </style>
</head>

<body>
  <div>
    <a href="http://baidu.com" target="iframeBox">baidu.com</a>
    <a href="http://so.com" target="iframeBox">so.com</a>
    <a href="https://www.sogou.com/" target="iframeBox">sogou.com</a>
  </div>
  <iframe src="http://baidu.com" class="iframeBox" name="iframeBox"></iframe>
</body>

</html>